1. Field of the Invention
The present invention relates generally to an expander cycle rocket engine, and more specifically to an expander cycle rocket engine with a gas generator.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The expander cycle is a power cycle of a bipropellant rocket engine meant to improve the efficiency of fuel delivery. The expander cycle rocket engine is shown in FIG. 1. In an expander cycle, the fuel is heated before it is combusted, usually with waste heat from the main combustion chamber. As the liquid fuel passes through coolant passages in the walls of the combustion chamber, it undergoes a phase change into a gaseous state. The fuel in the gaseous state expands through a turbine using the pressure differential from the supply pressure to the ambient exhaust pressure to initiate turbopump rotation. This can provide a bootstrap starting capability as is used on the Pratt & Whitney RL10 engine. This bootstrap power is used to drive turbines that drive the fuel and oxidizer pumps increasing the propellant pressures and flows to the rocket engine thrust chamber. After leaving the turbine(s), the fuel is then injected with the oxidizer into the combustion chamber and burned to produce thrust for the vehicle.
FIG. 2 shows the prior art RL-10 engine with a gear 3 to drive the LOX pump 4 off of the LH2 turbopump 5 drive shaft 3. Liquid hydrogen from the LH2 pump 5 is passed through a heat exchanger in the nozzle and vaporized, and then passed through a turbine 6 that drives both pumps. Because of the single shaft used in the FIG. 1 design, there must be an inter-propellant seal between the two liquids (the fuel and the oxidizer) to prevent mixing. These have historically been a design challenge due to the high cost of failure for the component.
Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of a bell-shaped nozzle increases with increasing thrust, the nozzle surface area (from which heat can be extracted to expand the fuel) increases as the square of the radius. However, the volume of fuel that must be heated increases as the cube of the radius. Thus there exists a maximum engine size of approximately 300 kN of thrust beyond which there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps. Higher thrust levels can be achieved using a bypass expander cycle where a portion of the fuel bypasses the turbine and or thrust chamber cooling passages and goes directly to the main chamber injector. Aerospike engines do not suffer from the same limitations because the linear shape of the engine is not subject to the square-cube law. As the width of the engine increases, both the volume of fuel to be heated and the available thermal energy increase linearly, allowing arbitrarily wide engines to be constructed. All expander cycle engines need to use a cryogenic fuel such as hydrogen, methane, or propane that easily reach their boiling points.
Some expander cycle engine may use a gas generator of some kind to start the turbine and run the engine until the heat input from the thrust chamber and nozzle skirt increases as the chamber pressure builds up.
In an open cycle, or “bleed” expander cycle, only some of the fuel is heated to drive the turbines, which is then vented to atmosphere to increase turbine efficiency. While this increases power output, the dumped fuel leads to a decrease in propellant efficiency (lower engine specific impulse). A closed cycle expander engine sends the turbine exhaust to the combustion chamber.
The gas generator cycle is a power cycle of a bipropellant rocket engine. FIG. 3 shows a gas generator cycle rocket engine. Some of the propellant is burned in a gas-generator 4 and the resulting hot gas is used to power the engine's pumps. The gas is then exhausted. Because something is “thrown away” this type of engine is also known as open cycle.
There are several advantages to the gas generator cycle over its counterpart, the staged combustion cycle. The gas generator turbine does not need to deal with the counter pressure of injecting the exhaust into the combustion chamber. This allows the turbine to produce more power and increase the pressure of the fuel and combustion chamber, thus increasing specific impulse or efficiency; this also reduces wear on the turbine, increasing its reliability, reducing its production cost and increasing its operational life-span (particularly advantageous for reusable rockets).
The main disadvantage is lost efficiency due to discarded propellant, though this efficiency loss can be outweighed in production engines by the higher chamber pressure's increase in net efficiency. Even so a gas generator cycle tends to have lower specific impulse than a staged combustion cycle.
As in most cryogenic rocket engines, some of the fuel in a gas-generator cycle is used to cool the nozzle and combustion chamber. Current construction materials cannot stand extreme temperatures of rocket combustion processes by themselves. Cooling permits the use of rocket engines for relatively longer periods of time with today's material technology. Without rocket combustion chamber and nozzle cooling, the engine would fail catastrophically.